When drilling a new borehole or a lateral in an existing well, the drilling head may drill into a very low pressure zone, resulting in a loss of pressure in the borehole. Thus, the mud entered into the hole while drilling to prevent blowout is lost in the low pressure zone, and there will be a substantial risk of a blowout if the drilling is continued. Cementing and thus sealing part of the annulus above the low pressure zone is also impossible, since the injected cement is lost in the same way as the mud as it disappears into the low pressure zone, and then this partly drilled borehole is abandoned and plugged from above and a new well is drilled.
Expanding and thereby setting an annular barrier in such a very low pressure zone by pressurising the inside of the well tubular structure opposite the annular barrier may not be successful. This is due to the fact that the expansion opening in the well tubular structure needs to be closed off after expansion, and the prior art closing mechanisms either close before the annular barrier is expanded or do not close at all due to the lack of pressure in the annulus. The prior art closing mechanisms rely on the annulus pressure to shift valve position and close the expansion opening and open fluid communication to the annulus, and when the annulus pressure suddenly becomes so low as is the case when drilling into the very low pressure zone, the pressure in the annulus cannot build up to a level high enough to shift the position of the closing mechanism which is normally done just after expansion or at least when equalisation of the pressure between the annulus and the inside of the annular barrier is required. Another risk is that the closing mechanism closes before the expansion has ended as some closing mechanisms rely on a shear pin to break after expansion, and since such shear pin is selected so that it breaks at a certain differential pressure and not at an absolute pressure, and the shear pin is selected so that it breaks when the pressure inside the well tubular structure is above a predetermined expansion pressure, the shear pin may break in the initial phase of the expansion process when the annulus pressure is very low, creating a high pressure difference across the shear pin so that it will shear too early and thus close the expansion opening before the expansion has even started.